The present invention relates to methods and apparatus for well drilling and more particularly concerns a hammer for driving casing while permitting access to the casing interior.
Many types of wells are lined either partially or completely with well casing. In the drilling of such wells it has been the practice to carry out the steps of drilling and lining alternately. Thus a drill bit is driven into the ground to remove material from the well, the drill bit is then removed and a section of casing sunk into the well. Then the drill bit is again inserted to drill another section of well which is lined after removal of the drilling tool.
In the drilling of water wells there has been adopted a method of drilling in which the casing is driven at the same time that the well is being drilled, with air forced downwardly through the drill string to remove loose material. Apparatus for carrying out such combined well drilling and casing driving is described in a U.S. Pat. No. to Back 3,833,072 and in patents to P. M. Cook 3,474,870 and Re. 28,151 and A. B. Cook 3,895,680.
In the P. M. Cook patents a heavy, hollow ram is raised by a pair of cylinders mounted on the outside of the ram in an arrangement in which the large heavy ram of relatively long stroke requires a relatively long hammer frame. The addition of such a long hammer to many existing drilling rigs requires that the rig frame be lengthened considerably in order to accommodate both the extra length of the hammer and standard lengths of twenty or twenty-five foot pipe. The arrangement of P. M. Cook, employing a relatively massive ram and imparting energy that is obtained mainly from the fall of the ram, is a relatively slow acting device, being operated at the rate of approximately one blow per second in a common operation. The force of the blow in the P. M. Cook apparatus, being largely determined by the weight of the ram, is not readily adjustable.
The A. B. Cook patent is subject to similar problems, using a ram considerably larger than the driving piston and thus, because of the large mass of the ram, severely limiting its rate of reciprocation.
It is frequently desirable to increase or decrease the force of each blow struck upon the pipe in accordance with the particular medium through which the pipe is being driven, the size of the pipe, and other conditions at a given well site. Such adjustment is not readily or conveniently available with the apparatus such as shown in the Cook patents where the fixed weight of a massive ram is a significant factor in the force of each blow.
In the patent to Back U.S. Pat. No. 3,833,072, instead of relying upon the weight and long stroke, the apparatus is driven by hydraulic pressure described as being 2,000 pounds per square inch, providing an impact energy of 4,765 foot pounds, at a frequency of thirty strokes per minute. Thus again a relatively low rate of exceedingly high energy blows is employed. These high energy blows are destructive to the driven pipe and its connections and to the hammer itself. All of the impact energy in the Back patent is transmitted directly to the hammer cylinder, which acts as the anvil, and thus the entire hammer receives the energy of each blow. Not only is this detrimental to the life of the hammer, but it creates considerable inefficiency since the entire hammer must be accelerated upon each blow and much of the kinetic energy of the piston is absorbed by the mass of the hammer. Thus the prior art teaches use of a large, massive device to either develop the impact energy or to transmit the impact energy so that the apparatus is either slow acting, inefficient, or both.
Accordingly it is an object of the present invention to provide a pipe hammer that eliminates or minimizes above-mentioned disadvantages.